The present invention relates to a power preservation system for volatile memory with nonvolatile backup memory, and, more particularly, is directed to a system for providing temporary power during unintentional outages of externally supplied power.
In military aircraft, electric power is provided for electric utilization equipment, such as a host computer. The aircraft electric power system includes a main power source derived from aircraft generators driven by the aircraft engines, an emergency power source such as batteries, power conversion equipment, an interconnection network and so forth.
When the electric power system transfers between power sources, power outages and transients may occur. In fact, due to vibration, outages and transients occur very frequently. This causes serious problems.
One such problem is that, unless special precautions are taken, data stored in the aircraft computer memory will be lost due to such power outages. Maintaining the reliability of this information is a serious problem because the data is crucial to flight of the aircraft, operation of weapons and so forth.
One conventional technique for preventing loss of data due to power outages in aircraft is to use magnetic core memory as the memory for the computer. Advantages of core memory include insensitivity to power outages, robustness in a harsh environment, operability over a wide temperature range, no wear problem (long life), and a relatively fast access time, such as one microsecond. Disadvantages of core memory include its relatively large physical size, such as 6".times.9".times.1.4" for 256 kilobytes, its relatively high power usage, such as forty watts when accessed, destructive read-out (all core memory used in aircraft is of this type), poor availability of small geometry ferrite cores, and difficulty in providing faster access times as required by more modern aircraft.
Another conventional technique, typically used in spacecraft, is to use a plated wire memory. In this scheme, wires coated with magnetic material are laid in parallel, and perpendicular to the wires are wide, flat conductors functioning as word straps. Disadvantages of plated wire memory include very high cost, very large physical size (substantially larger than core memory) and lack of ready commercial availability.
Volatile random access memory (RAM) with battery backup has been used in some military aircraft. However, the batteries have proven to be unreliable, resulting in unacceptably poor aircraft readiness.
Another prior art technique is to use static RAM as the working memory for the host computer, and to transfer the data from static RAM to electronically erasable programmable ROM chips (EEPROMs) when a power outage occurs, and then transfer data back to the static RAM after the outage ends. This technique has at least two problems.
First, while a data transfer is occurring, the working memory is not available to the host computer. Since transfers may occur at critical points in the aircraft flight mission and since the transfers may take a substantial length of time, such unavailability is unacceptable.
Second, an EEPROM has a limited number of storage cycles; that is, an EEPROM wears out after a given number of storage cycles. Because of the frequency of power outages in aircraft, the limit easily can be exceeded over the lifetime of an aircraft. Furthermore, there is no convenient way of checking how many storage cycles a particular EEPROM has actually experienced. Consequently, it is necessary to replace the EEPROMs frequently to ensure that they will be operable when needed. This creates a delicate maintenance requirement that is difficult to perform properly under expected usage conditions of the aircraft.